The present invention pertains in general to color picture tubes having the shadow mask for color selection. In particular, the invention relates to a method for protecting shadow masks during their manufacture, especially those of the high resolution type.
A color cathode ray tube that utilizes the shadow mask typically includes three electron guns arranged in a delta or inline configuration. Each gun projects an electron beam through the assigned apertures of a shadow mask, also called a "color selection electrode," onto a target area on the inner surface of the faceplate. The target area comprises a pattern of phosphor deposits arranged in groups of triads of dots consisting of a dot of a red-, green-, and blue-light-emitting phosphor. To improve the brightness of the display, and to minimize the incidence of color impurities which can result if a beam falls upon an improper phosphor deposit, the target area may include a layer of a darkish light-absorbing material called a "grille" that surrounds and separates each of the dots. This type of screen is known as a "matrix" or "black surround" screen. Alternately, the phosphor and grille deposits on the target surface may comprise a plurality of vertically oriented, spaced rectangles in coordinate relationship to apertures in the for of rectangles or "slots" in the shadow mask. Tubes of this type are referred to as "slot mask tubes," in contrast to the "dot screen" types of tubes.
The phosphor pattern screened on the faceplate, whether dots or stripes, (depending on the type of mask), is typically formed by a direct photoprinting process. The target area is first coated with a photosensitive slurry comprising phosphor particles of one of the three phosphors described. The shadow mask, mounted in a frame, is temporarily installed in precise relationship to the faceplace, and the coating is exposed to actinic light projected through the apertures of the mask from a light source located at a position that corresponds to the beam-emission point of the related electron gun. The faceplate is separated from the shadow mask and the coating is "developed" to remove unexposed portions. The result is a pattern of dots or stripes capable of emitting light of one color, whether red, green or blue. The mask is then re-registered with the faceplate, and the steps are repeated for each of the remaining colors to deposit triads of phosphor deposits on the target area on the faceplate in coordinate relationship with each aperture of the mask. A further step, usually taken before the deposition of the phosphors, is the application of the black surround.
A major problem of the manufacture of the conventional domed shadow mask is that of the plugging of one or more of the 400,000 odd apertures in the mask. Aperture diameter is about six mils and particulate matter in the form of dust floating in the air can settle on the mask surface during production and become embedded in an aperture. Such particles can be dislodged in most cases by blowing air through the apertures, or by vibrating the mask. A particularly pernicious offender is a contaminant in the form of lint which, upon entering the aperture can "hook" itself into the aperture. Lint attached in such a manner is usually immune to dislodgement by forced air or by vibration. A single blocked aperture in the corners of a commercial television tube usually will not be noticed by the viewer; however, if the blocked aperture is near the center of the tube, the omission will be readily apparent and will result in a customer complaint.
In the manufacture of flat-faced high-resolution tubes having the tensed-foil, planar shadow mask, aperture-occlusion is a particularly severe problem. The apertures are smaller in diameter--e.g., 0.0036 inch--and there may be as many as a million of them. The clogging of even one aperture is cause for rejection of the tube. As a result of this problem, the yield in manufacture can be seriously affected. Clogging that may not be noted with the unaided eye during manufacture can occur at any of the many stages of screening of the target area of the faceplate. For example, the target area may have been successfully screened to receive the black surround and the red and green phosphor deposits. However, a floating particle may lodge in an aperture before the final screening of the target area in which the blue-light-emitting phosphor is to be deposited. The omission of a "blue" phosphor dot will become apparent in the final quality check. As a result, all of the deposits will have to be removed from the target area and the faceplate and panel will have to be recycled through the entire process. The severity of the problem can be comprehended in view of the fact of the high probability of one aperture out of a million being accidentally occluded by a vagrant particle. Unless measures are taken to prevent such aperture occlusion, the manufacturing yield will be prohibitively low.
By way of example, a shadow mask blank for a high-resolution color cathode ray tube typically goes through at least 24 distinct process steps from manufacture until the time of its final installation in the tube envelope. The manufacturing process includes rolling, annealing, forming, screening and perforating. The blanks are processed in near-final form in that they have about one million color selection apertures, plus auxiliary apertures for mounting and trimming. From this stage until the final enclosure in the tube envelope, the blanks are most vulnerable to particle occlusion. The blanks are handled by the mask blank manufacturer during outgoing inspection, and packaging for shipping. When received by the tube manufacturer, the blanks go through incoming quality control and are otherwise prepared for installation in the tube. This preparation typically includes the steps of mask pre-tensing and tensing, cement deposition, setting of the cement, cleaning, screening in conjunction with the faceplate--a process which alone takes at least four separate and distinct steps--and finally, installation of the framed mask into the tube and sealing into the tube envelope. At any point during these many procedures, dust and lint can invade to occlude one or more apertures.
In U.S. Pat. No. 3,935,036, Kinsch discloses a method of forming a dark, very adherent coating on the metal of a CRT mask assembly. During manufacture, the coating serves by its adherency to preclude occlusion of the nearby apertures due to flaking of the coating during the screening process. The coating is baked on at about 435 degrees centigrade. During tube operation, the coating functions both as a heat radiator and an absorber of light which could otherwise reflect from the mask surface and wash out the picture.
Filming compounds for use in manufacture of various coating substrates include anti-static formulations which are usually rolled or sprayed on. A typical application is the use of a gelatin-based anti-static film which is applied to the back surface of a web of drafting film to prevent static build-up as the web is conveyed at high speeds. Other thin film applications include anti-static sprays for lens surfaces. The anti-static additive is typically an optically clear liquid that is enough of an electrical conductor or a surface lubricant to bleed off a static charge. A well-known compound of this type is GAFAC RE-610 manufactured by GAF Corporation.
As used in the semiconductor substrate manufacturing industry, a "pellicle" is a container for enclosing the semiconductor during the screening process. The top of the container comprises an optically clear film having a thickness in terms of microns. The surface of the semiconductor is spaced a predetermined distance from the surface of the film, with the result that during the screening process, any dust particles that fall on the film are effectively outside the depth of field of the lens used in the screening process. The base for the optically clear film may be by way of example, a nitrocellulose or cellulose acetate butyrate.